The invention relates to the fatigue testing of mechanical components, notably engine or motor components.
These fatigue tests are often carried out with mechanical machines comprising cams, excentrics, linkages, cranks and rams working at limited frequencies and requiring very rigid and heavy frames in order to apply the high forces needed.
Another solution consists in exciting the element under test at its natural frequency by an exciter or high-frequency ram, which enables the testing time and forces applied to be reduced since maintenance of the oscillations only needs the energy loss absorbed by friction and internal damping to be made up, and these are low in metals. Owing to this low damping the overtension factor at resonance is very high, which means that the exciting device only needs to supply a small force to maintain large movements corresponding to considerable stresses.
Consequently an exciter is used driven by a generator the frequency of which is tuned to that of the element under test.
In particular electromagnetic vibrators are easy to use and can reach much higher frequencies than the previous mechanical machines, but the efforts they supply are very limited so that if the natural frequency of the element under test varies due, for example, to heating, work hardening, incipient failure or any other reason, so that it differs from that of the generator, the effort required of the vibrator soon becomes too large to maintain the movement amplitudes and the stress levels. It is therefore necessary to constantly retune the generator frequency to the natural frequency of the element under test, and this constitutes a problem which has not found a satisfactory solution at the present time.
In addition, during the endurance test, it is necessary to control the electrical level of the signal so that the effective amplitude of the mechanical vibrations of the element under test are maintained at a preset value corresponding to constant deformations and stresses characteristic of the endurance test.
Consequently the aim of the invention is to solve this problem as simply and automatically as possible, i.e. to produce an electric generator with a sinusoidal frequency, the frequency of which is at all times equal to the natural frequency of the element subjected to the tests and the amplitude of which is constantly controlled, so that the natural amplitude of the mechanical vibrations of the element subjected to the tests is equal to a predetermined setpoint value.